Many currently successful chemotherapy agents have been derived either directly or indirectly from plant material, and it is highly likely that additional useful anti-tumor agents remain to be found. The montane cloud forest in Monteverde, Costa Rica, the tropical rain forests in North Queensland, Australia and the tropical forest on Abaco Island, The Bahamas, contain extremely biodiverse populations of plant species, most of which have not been tested for any biological activity, let alone anti-tumor activity. The collection and testing of plant species from these locations for several years has already led to the isolation and identification of several compounds cytotoxic to human cancer cell lines. Based on previous discoveries of useful breast cancer drugs from plant material, we hypothesize there are additional novel and undiscovered compounds in tropical plants which will be useful chemotherapy agents against breast cancer. The primary objective of this proposal is to search a broad spectrum of previously untested tropical plants for new potential chemotherapeutic drugs for breast cancer. The specific aims are (1) Use a 96-well cytotoxicity assay against a panel of selected breast cancer cell lines to screen plant extracts for those with potent inhibition or killing of cells; (2) Use an innovative high-throughput genetic-based bioassay to screen the cytotoxic extracts against a panel of mutated yeast strains with genetic defects in DNA repair pathways and/or cell cycle regulation and/or growth factor signaling. (3) From the results of these screens, identify those extracts most promising for containing new anti-tumor compounds. (4) Fractionate the active extracts, isolate and purify the active compounds and determine their structures. (5) Assess the in vivo activity of these compounds against human breast tumor xenografts in nude mice. (6) Investigate the mechanism of action by which these compounds kill or inhibit the tumor cells. The active chemicals isolated will provide either new drugs or new starting materials for chemical modification to produce new drugs. This research may lead to new breast cancer chemotherapeutic agents with novel structures and/or mechanisms of action.